The separation of air into constituent components such as oxygen and nitrogen has been practiced for many years utilizing energy intensive processes such as cryogenic distillation, adsorptive separation, chemical absorption, and differential permeation through membrane media. These processes generally suffer from high utility costs or the lack of continuous or convenient regeneration or operation.
Various processes for separating air have been suggested in which at least some power is recovered from an effluent stream to provide a portion of the power requirements of the process itself.
For example, U.S. Pat. No. 4,132,766 describes a chemical air separation process wherein air, at elevated temperature and pressure, is separated into oxygen and nitrogen-rich waste streams by preferential chemical binding of oxygen from the air. The nitrogen-rich stream is then reduced in pressure through an expander before being vented. The expander drives the compressor to recover some power for the process. No net power is achieved by the process.
To provide additional power for the operation of compression equipment in a gas separation process, U.S. Pat. No. 4,340,578 suggests that in a chemical air separation plant, the waste nitrogen stream, still containing residual oxygen, is combusted with a fuel. The hot effluent is then expanded in several stages through turbines and power is recovered.
Alternately, U.S. Pat. No. 4,560,394 discloses that air may be compressed in a compressor, reduce in temperature by heat exchange against process streams or external cooling means and then separated into oxygen and a nitrogen-rich effluent stream by passage over a semipermeable membrane. While some power is recovered by pressure reduction of the nitrogen-rich stream, no fuel is combusted and no net power is produced.
Power generation can be achieved using a cryogenic air separation process as described in U.S. Pat. No. 4,224,045. In that process, air is compressed and then cooled to its liquefaction temperature before being distilled through a fractionation column. The waste stream from the column is rewarmed, recompressed, and then combusted with fuel and by-pass air. The combusted effluent is expanded through a turbine to recover power for the process.
U.S. Pat. No. 4,545,787 teaches a method for the generation of net power and the recovery of by-product oxygen-rich gas at low power requirements. Air is compressed to an elevated temperature and pressure. At least a portion of the air is combusted and a portion of the oxygen is removed from the air or combustion effluent through a membrane or adsorbent before the oxygen-lean combustion effluent is expanded through a turbine to recover power for the process and net power.
In the '045 and '787 patents discussed above, the type of fuel used is generally limited to "clean fuels" such a natural gas, oils, or synthesis gas in order to prevent the gas turbine blades and associated equipment from corroding and eroding. This rules out the use of cheaper, more abundant fuels such as coal, coal slurry, lignite, petroleum coke, biomass and solid waste. In an effort to overcome this disadvantage in the use of gas turbine systems for power generation, an externally fired gas turbine/combine cycle (EFGT) has been proposed as discussed, for example, by LaHaye et al. in Externally Fired Gas Turbine/Combine Cycle, Coal Technology, 1986 and Eberhardt et al. in U.S. Pat. No. 4,761,957.
In the EFGT power generation process, air is compressed and heated by a ceramic heat exchanger using an external hot combustion gas source. The clean hot air is then expanded through the turbine to recover power. The exhaust air is then fed to a suitable atmospheric combustor. The hot combusted effluent gas is used to heat the compressed air passing through the heat exchanger. The hot gas leaving the heat exchanger can further feed a steam generation (hence, combined cycle) system. Because the gas turbine blades see only clear hot air instead of combustion gas, the service life of the turbine is extended appreciably. However, the EFGT/combine cycle produces no oxygen.